Marron, freshwater crayfish Cherax tenuimanus - Department of ...

consequences; possible destruction of adjacent terrestrial vegetation in the riparian zone, a small
but possible disturbance to breeding of bottom-spawning fish, introduction of associated
undesirable parasites, and lastly far-ranging but slight impacts on benthic macroinvertebrate
communities the degree to which would be dependent on the population size of C. tenuimanus. It is
not likely to hybridize with any indigenous species as there are no native freshwater African crayfish
species (de Moor 2002).
The existing broodstock of C. tenuimanus in South Africa has been used for several years and as a
result, the industry requires new blood lines to prevent inbreeding and associated genetic issues (D.
Impson, Cape Nature, pers. comm.). If new stock is imported from Australia, there are risks of
disease or parasite introduction which must be considered.
A significant impact which must be considered is the threat of introduction of undesirable parasites
with C. tenuimanus. The ‘crayfish plague’ fungus, Aphanomuyces astaci can affect all species in the
Parasticidae family (as well as other freshwater crayfish families (de Moor 2002). Also, C. tenuimanus
is host to a microsporidian parasite, Thelohania sp., commonly known as ‘porcelain disease’
(Morrissy et al. 1990, Langdon 1991) which affects striated muscle fibres in the tail. There is no
treatment for this disease and the only means of prevention is by ensuring that stocks are disease
free. The disease is, however, extremely difficult to detect during the early stages of infection, so the
prevention of its importation is likely to be difficult (Langdon 1991). However, this disease has not
been reported in South Africa to date (DAFF 2012b) and is unlikely to affect any indigenous
freshwater species.
There are various worms which are hosted by crayfish without causing the host any harm, but have
the potential to infect other species. For example, temnocephalan worms (which are non-native to
Africa) can infect other decapods species and can predate on freshwater invertebrates.
Temnocephala chaeropsis was introduced with C. tenuimanus in the southern Cape and affected the
marketability of infected individuals, in some cases causing mortalities (Mitchell & Kok 1988). The
worms also have the potential to infect a species of indigenous freshwater crab, Potomonautes
warreni (Avenant-Oldewage 1993). In addition, T. chaeropsis could affect other indigenous
decapods, either by killing or by lowering its fitness to a level which would allow C. tenuimanus a
competitive advantage (De Moor 2002).
10.3 Potential socio-economic impacts
Currently there are no commercial freshwater fisheries in South Africa, and subsistence reliance on
freshwater crustaceans is extremely low (B. Clark Anchor Environmental, pers. comm.), so neither of
these fisheries should suffer significant impacts as a result of further introductions of C. tenuimanus.
It is also unlikely that recreational fisheries will be negatively affected.
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